Techniques currently utilized in printers and the like for transferring an image to a recording medium, e.g., paper, based on electrical or optical signals include dot-impact printing, thermal transfer printing, thermal sublimation printing, ink-jet printing, and electrophotography in laser printers. These techniques are roughly classified into three groups.
One group includes dot-impact printing, thermal transfer printing, and thermal sublimation printing. In these techniques, a sheet containing dye molecules dispersed therein, e.g., an inked ribbon or a donor film, is superposed on paper or the like, and the dye is transferred to the paper by means of mechanical impacts or heating. These techniques therefore have drawbacks that expendables are always necessary, that it is difficult to increase the printing speed, and that the printing has a low energy efficiency and high running cost. Moreover, the prints obtained with these techniques excluding thermal sublimation printing have poor quality.
On the other hand, ink-jet recording, which is included in another group, has an advantage that running cost is low because an ink is directly transferred from heads to paper and expendables other than an ink are hence unnecessary. However, it is difficult to increase the speed of ink-jet printing, because all dots are formed with electrical control and because of difficulty in fabricating an array of heads having the width of the paper. Another drawback of ink-jet printing is that since the minimum image unit is determined by head size and head interval, higher print quality results in lower printing speeds and lower energy efficiencies.
Electrophotography, which is included in the remaining group and is used in laser printers, etc., is a technique of forming an image through an intermediate transfer medium. In electrophotography, a toner is adsorbed onto an electrostatic image formed on a photoreceptor by laser spots, and the adsorbed toner is transferred to paper to form an image. Electrophotography can hence form relatively fine images. In addition, it has an advantage of low running cost because a toner is the only expendable. However, electrophotography has problems that the consumption of electrical power is large because of the necessity of a high voltage for the formation of latent images and for the adsorption and transfer of a toner, and that electrophotographic apparatuses generate ozone and nitrogen oxides. All the printing techniques described above further have a problem of a considerably loud printing noise.
On the other hand, among the image-forming techniques which give high-quality images are conventional printing techniques using a printing plate and silver halide photography. However, the conventional printing techniques have a drawback that these are unsuitable for general applications because of the necessity of forming a plate, although the running cost thereof is low when the same image is formed in a large quantity. Silver halide photography and the like have drawbacks that because of the necessity of using media which are not reusable, such as photographic films and photographic printing paper, the running cost thereof is high and an increase in printing speed is not expected.
As described above, any of the image-forming techniques currently utilized in printers and the like is not a printing technique which gives high-quality images, attains a relatively high printing speed and a low running cost, is energy-saving and resource-saving, and is environmentally friendly and user friendly.
One means for eliminating the above-described problems may be to utilize a medium with which an image distribution corresponding to the image to be printed is formed with an image-forming element, e.g., a toner or an ink, on the same scale (the same paper width) as on the receiving material and is transferred indirectly or directly. This medium, which functions as a temporary holder for an image-forming element, is required not only to attain a relatively small energy consumption and continuous tone in the incorporation and release (transfer) of the image-forming element but also to be capable of coping with size reduction in units of the image-forming element.
Examples of media which can satisfy such requirements include films of conducting polymers represented by polypyrrole, polythiophene, and polyaniline. It is known that films of these polymers can be chemically, electrically, or electrochemically regulated so as to come into any of three states, i.e., oxidized, neutral, and reduced states, and these state changes are accompanied by doping with and undoping of counter ions. Such properties are described in detail in, e.g., Susumu Yoshimura, "Do densei Porima (Conducting Polymer)" (The Society of Polymer Science, Japan); Kazuo Yamashita and Hiroshi Kitani, "Do densei Yu ki Hakumaku No Kino To Sekkei (Function and Design of Electroconductive Organic Thin Film)" (The Society of Surface Science, Japan); and Katsumi Yoshino "Dodensei Kobunshi No Kiso To Oyo (Fundamental and Application of Conducting polymer)" (IPC).
To sum up, a conducting polymer thin film capable of being doped with dye molecule ions and undoped to release the ions is expected to function as a temporary image-forming-element holder which satisfies the requirements described above. However, the counter ions with which conducting polymers are doped and undoped have generally been limited to the anions and cations of general metals and small molecule electrolytes. It is known that in the case where a conducting polymer is synthesized in the presence of, e.g., high molecular anions or the like, the resulting polymer cannot be undoped.
On the other hand, the size of ions with which a conducting polymer film can be reversibly doped and undoped is determined by the microstructure of the film. It has been reported in Hiroaki Shinohara et al., J. Chem. Soc., Chem.
Commun., p. 87 (1986) that the size of those ions can be controlled, for example, by regulating the size of counter ions in the presence of which a monomer is polymerized to produce the conducting polymer. However, the molecular weights of the ions investigated in the above report are up to about 100, and the results given therein show that the higher the molecular weight, the poorer the doping/undoping characteristics. Although an example of reversible doping/undoping with relatively large molecules has been reported by the same investigators including Shinohara in Journal of Chemical Society of Japan, No. 3, p. 465 (1986), this investigation was made with glutamic acid, whose molecular weight is below 150. On the other hand, many generally employed dyes have a molecular weight in the range of about from 500 to 1,500; it has hitherto been thought that conducting polymer films cannot be reversibly doped with and undoped of ions having such a high molecular weight.
Prior art applications of conducting polymer films based on such doping/undoping with low-molecular weight ions and on the accompanying color changes have been directed mainly to protective films for batteries and solar cells and to electrochromic display elements. On the other hand, use of a conducting polymer film as a material for marking is disclosed in JP-A-2-142835, "Method for Controlling Wettability of Surface of Thin Polymer Film and Method and Material for Image Formation Based on that Method." (The term "JP-A" as used herein means an "unexamined published Japanese patent application.") However, this prior art technique has a drawback that since a printing plate is formed by changing the wettability of a conducting polymer film by means of electrical shifting between an oxidized state and a neutral state, the conducting polymer thin film neither functions to keep a dye therein through doping, nor is regulated at all with respect to the adsorption or transfer amount of a dye, e.g., an ink.